A Wet and Warm Spring, Then 200,000 Dead Saigas

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A field of dead saigas in Kazakhstan in May 2015. The die-off of the endangered antelopes was found to be caused by bacteria that somehow became harmful after particularly warm and wet weather.CreditCreditAssociation for the Conservation of Biodiversity, Kazakhstan, Biosafety Institute, Gvardeskiy RK, Royal Veterinary College, London, UK

Among saiga antelopes, the month of May ought to be about new life. But in 2015, it was just the opposite for the Betpak-Dala saiga population in central Kazakhstan.

In only three weeks, about 220,000 of the critically endangered antelopes, most of them newborns and mothers that had gathered to calve, dropped dead across an area the size of Britain.

In a study published today in Science Advances, researchers presented a preliminary account of the cause of the mysterious die-off: Bacteria called Pasteurella multocida, which seem normally to exist harmlessly in saigas’ tonsils, somehow invaded their guts, poisoning their blood and breaking down their organs, leading to death within a few hours.

The mechanism that allowed the bacteria to become so harmful is still unclear, but the scientists believe it had something to do with a peculiarly wet and warm period before the outbreak.

“One possible explanation might be that climate change is driving these events,” said Richard Kock, a professor at the Royal Veterinary College in London and an author of the paper. There have been a series of such die-offs in recent years, he noted, but the team found no evidence for these events before the 1980s.

Scientists now need to model how climate change might affect saigas in the future, particularly if unusually wet and warm weather events become more common in their range.

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A newborn saiga held by a researcher in Kazakhstan.CreditAssociation for the Conservation of Biodiversity, Kazakhstan, and Royal Veterinary College, London

In the new study, Dr. Kock and his colleagues first confirmed that P. multocida was the immediate cause of death in 2015. They ruled out other pathogens and toxins (including Russian rocket chemicals, as a handful of ecologists had speculated). They found nothing significant in the soil or the vegetation the antelopes had been exposed to, and determined that the animals were not nutritionally deficient or immunosuppressed.

What did show a strong correlation with the die-off in 2015 — as well as two similar events in 1981 and 1988 — were the average relative humidity and average minimum temperatures in the 10 days leading up to mass mortality. During these 10 days, if relative humidity is greater than 80 percent, there’s a strong possibility of outbreak, said Wendy Beauvais, a postdoctoral student in veterinary medicine at Cornell University and an author of the study.

It’s clear that an environmental trigger allowed the bacteria to wreak havoc, but how remains a mystery, Dr. Kock said. The 100 percent fatality rate that occurred in 2015 was unprecedented among similar outbreaks in other large mammals.

“I’ve worked with many nasty things,” he said. “You always get survivors.”

Another contributing factor may be the saiga’s unique life history. Since females give births to unusually large calves, they tend to be stressed and more susceptible to disease during calving season.

Furthermore, the antelopes have evolved specific mechanisms to deal with cycles of weather extremes in the Eurasian steppe, including their Seussian trunks, which facilitate heat exchange and keep out dust. The more “fine-tuned” an animal is to specialized conditions, the more vulnerable it might be to change, Dr. Kock said.

Going forward, conservationists perhaps won’t be able to stop environmental change, but they can try to make sure the world’s five saiga populations are resilient enough to cope with disease outbreaks, said E.J. Milner-Gulland, a professor of biodiversity at Oxford and an author of the paper.

This means managing other threats, such as encroachment on the saigas’ habitat and poaching. “Saigas have a history of bouncing back,” she said, “so we always have hope.”

A version of this article appears in print on , on Page D2 of the New York edition with the headline: Deadly Bacteria: During a Warm Spring, 200,000 Antelopes Die. Order Reprints | Today’s Paper | Subscribe